snd_motu: add checks for method arguments
snd_motu: add checks for method arguments

In Rules for use of GError, it's just used for recoverable runtime
error, not for programming error. The invalid arguments are a kind of
programming error.

This commit adds the check for method arguments.

Reference: https://developer.gnome.org/glib/stable/glib-Error-Reporting.html
Signed-off-by: Takashi Sakamoto <d5ec7d0cd8074757a1f5eb708bc105c3065a65ea@sakamocchi.jp>

takaswie/libhinawa,takaswie/libhinawa